1. Field of the Invention
This invention involves a structural non-destructive control system and in particular, a system for characterizing structural damage from observing surface distortions of said structure.
2. Discussion of the Background
Assessment of the structural damage, in other words, any structural problem that may affect its integrity, can be critical when used in severe environments. In the field of aeronautics for instance, impacts of miscellaneous objects on the structure of an aircraft can diminish mechanical features, especially, in the case of an impact at a sensitive location. Likewise, in the case of a distortion that took place during the manufacturing or assembly stages of a part, referencing of such defects is necessary.
When distortions are considerable, their characterization can be done directly by an inspection operator who is making a decision about what is to be done with the damage. Such decision generally involves making a choice between validation, repair or, in the most unfavorable cases, replacing the defective part.
When it involves small distortions, viewing with the naked eye shows its limits and generally requires that the inspection operator is very well trained. Thrusting aside these limits is even more important in the case of small surface distortions when the internal degradation of the structure is considerable.
Traditionally, to correct this problem, surfaces are tested using mechanical sensors that enable measuring a distortion with respect to a theoretical profile. However, these mechanical means are very difficult to implement whenever one attempts to inspect pieces in operation. Indeed, these devices are often not very practical and fragile and as such, are incompatible with unprotected environments. In addition to this, one must consider the risk of damage during the transportation of these inspection devices, not taking into account that the sensors generally used require an extensive implementation and have a high usage cost.
Moreover, for quality reasons, one may want to arrive at more precise results than those obtained with classical sensors. Indeed, for a defect with a small diameter but of considerable depth, the inspector may incorrectly assess the damage while the structural fatigue risks are very much present and may be important according to the position of the defect on the structure.
To improve the observation of the distortions, the use of optical principles has also been known.
In document EP 0 674 151, a viewing of the surface defects is done by means of a shadow moiré on the observed surface. It permits to point out the existence of a distortion of the surface by observing through a rigid grid or screen the shadow of the network of this grid or screen on the surface, projected by a light source. Such a device equipped with a rigid grid or screen is only used overhead with respect to the surface and in addition, does not give any quantitative information about the extent of the distortion.
Document WO 00/39523 tries to provide a response to this last problem. This document shows an overhead moiré grid or screen with respect to a structural surface to be observed and proposes to measure the extent of the distortion of the generated pattern through the projection of the grid image on the surface. This solution then requires comparison of the scope of the distortion with the results of the measurements made on samples for which the deformity is known. To obtain comparable results with those obtained on calibrated samples, adjustment of the grid incline is essential.
A problem of these devices resides in the fact that the grid is overhead. In practice, placing such devices overhead is difficult and going across the surface with them becomes practically impossible in particular when an accurate measuring instrument, like a micrometer, is used to incline the grid or screen a certain angle for each measurement.
Another problem encountered with the process indicated by document WO 00/39523 resides in the fact that all types of distortions must be known because afterwards they are compared with calibrated samples to permit characterizing the observed surface problems. Within the framework of assessing cold compression of a metal such as the one presented in the document in question, the device seems appropriate but as soon as one goes beyond a specific application and one looks to assess any type of surface distortion, the device is not very useful.